They don't need all of them to do fill those spots, they need two of them to fill the top 2 spots. If Cease becomes a #4 it's ok. They can find a #3 starter elsewhere.
They need Giolito to, at worst, repeat his 2019 season for the foreseeable future and Kopech to become the ace quality pitcher we all think he can be.
Gio and Kopech are the Sox best bet for getting a true ace, and I wouldn't put it past either to pull it off. In a BEST case scenario, they both pull it off. I'm fairly confident that one of the two will reach ace level.
There's much more uncertainty with Cease, but he has that ability as well. I'm not sure the command ever gets there. I've seen more ability to hit spots out of Giolito and Kopech(in his limited sample size) than Cease.